Currently, separation membranes made of polymer membranes are used widely in various fields such as water treatment (e.g. for eliminating bacteria or viruses in water), medical treatment (e.g. for dialysis), and industry (e.g. for producing ultrapure water). The separation membranes are provided with continuous pores that communicate between both main surfaces of the membrane. Separation membranes can be classified by the pore diameter into microfiltration membrane (MF membrane), ultrafiltration membrane (UF membrane), nanofiltration membrane (NF membrane), reverse osmosis membrane (RO membrane) or the like.
Although use as a separation membrane is not necessarily intended, processes for producing a microporous polymeric object having continuous pores are disclosed in JP2(1990)-279741A (document 1), JP5(1993)-287084A (document 2), and JP11(1999)-80414A (document 3). In the processes of the documents 1 to 3, phase separation of polymer is used for forming the pores. Phase separation of polymer can be classified by its scale into microphase separation (also referred to as nanophase separation: wherein the scale of phase separation is less than 1 μm, typically several nm to several hundred nm), and macrophase separation (wherein the scale of phase separation is 1 μm or more, typically several μm to several ten μm).
As an example of microphase separation, there is a phase separation due to incompatibility between segments in a block copolymer, where various phase separation structures, such as globular structure, columnar structure, co-continuous structure, lamellar structure, or the like, are known to be formed depending on the composition of the copolymer. For the use as a separation membrane, it is preferable to use microphase separation where a co-continuous structure is formed, because such separation enables formation of winding pores that are continuous three-dimensionally. However, usually, the composition range of a block copolymer that allows such microphase separation is quite narrow. For example, H. Hasegawa et al., “Bicontinuous Microdomain Morphology of Block Copolymers. 1. Tetrapod-Network Structure of Polystyrene -Polyisoprene Diblock Polymers”, Macromolecules, vol. 20, p. 1651-1662 (1987) discloses that, in AB-type diblock copolymer made of two mutually-different segments, a co-continuous structure is formed only in a quite narrow range where one volume fraction thereof is 0.34 to 0.38.
In the process of the document 1, mixing a polymer (A) having a functional group (a) capable of ionic bonding at both terminals of the polymer chain and a polymer (B) having a functional group (b) capable of ionic bonding with the functional group (a), the polymer (B) and the terminals of the polymer (A) are ionic-bonded so as to form a pseudo-block copolymer, thereby allowing microphase separation between the region made of polymer (A) and the region made of polymer (B) and forming a polymer thin film having a microphase structure made of the polymers (A) and (B). After that, by breaking the ionic bond between the polymer (A) and the polymer (B) to extract and remove either the polymer (A) or (B) from the polymer thin film obtained by the phase separation, a microporous polymer thin film is obtained. The obtained polymer thin film is made of only either one of the polymers (A) or (B) that has not been extracted and removed. However, in this process, since the ionic bond developed between the polymers (A) and (B) is a bond based on the functional groups located at the terminals of the polymer chain, the bond ratio with the polymer is low. Further, the bonding force is smaller compared to a normal bonding force (shared bond) between segments in a block copolymer. That is, since the force and number of the bond developed between the two polymers are both small, it is difficult to cause microphase separation stably. Furthermore, practically, since it is not phase separation of a block copolymer but phase separation in a state where a plurality of the polymers (A) are bonding to a single polymer (B), formation of a co-continuous structure, which already occurs with difficulty in a block copolymer, may become even less likely to occur. Actually, in the document 1, only a lamellar structure is disclosed as a phase separation structure, while co-continuous structure is neither disclosed nor suggested therein.
The document 2 discloses a process for widening the copolymer composition range where a co-continuous structure can be formed due to microphase separation, in AB-type or ABA-type copolymer that is a two-component block copolymer made of two kinds of mutually-incompatible segments (in the document 2, which are referred to as “polymer chains” or “block chains”) A and B. In one of the processes disclosed in the document 2, a polymer having compatibility with the segment A is mixed to the copolymer (see e.g. Claim 4 in the document 2). In this process, the molecular weight distribution of the polymer to be mixed is required to be broad for swelling (which is carried out by the polymer with a relatively low molecular weight) and filling (which is carried out by the polymer with a relatively high molecular weight and with similar molecular weight to the segment A) of the segment A, when forming a co-continuous structure (see e.g. paragraph number [0029] and Example 4 in the document 2). Also, in the document 2, there is described that a microporous membrane with a three dimensional network structure can be obtained, by removing either one of the segments, by ozonolysis, in a block copolymer with a co-continuous structure formed due to microphase separation, or extracting the mixed polymer by means of a selective solvent.
According to the process of the document 3, a block copolymer having two or more kinds of segments and a first homopolymer are mixed, followed by a macrophase separation into the region made of the block copolymer and the region made of the first homopolymer. Subsequently, after the microphase separation of the block copolymer, pores are formed by elution of the first homopolymer, so that continuous pores are formed by further elution of one phase of the block copolymer remaining as a framework, or elution, from the block copolymer, of a second homopolymer added for controlling the final microphase separation as needed (cf. paragraph number
in the document 3). In the document 3, it is exemplified that, in order to elute one phase of the block copolymer, a block copolymer that is degradable by ozone or light is used, or a pseudo-block copolymer by ionic bond at the terminals of the polymer chain, as disclosed in the document 1, is used. However, in the process of the document 3, when using a pseudo-block copolymer by ionic bond as disclosed in the document 1, the same problem as described in the above document 1 may occur. Further, basically in this process, the diameter of the pores formed due to macrophase separation is too large to use the obtained porous polymeric object as a separation membrane.
In paragraph number [0011] of the document 3, there is described, “the present invention uses microphase separation structure of the copolymer, as disclosed in JP5(1993)-2807804A (which may be a clerical error for JP5-287084A), as well as macrophase separation between (the first) homopolymer mixed in the system and the copolymer, and thereby . . . ”. In view of this, the second homopolymer added for controlling the microphase separation may be equivalent to a polymer (including the polymer made of monomer units constituting the segment A) that has broad molecular weight distribution and compatibility with the segment A and that has been added for the purpose of swelling and filling of the segment A, in the document 2.
Although it has no direct relationship to formation of a microporous polymeric object, it is known that, in a three-component block copolymer where the contained monomer units are made of three mutually-different segments A, B and C, the composition range of the copolymer where a co-continuous structure is formed due to microphase separation is wider, compared to the two-component block copolymer where the contained monomer units are made of two mutually-different segments A and B (which is described in, e.g. Yasuhiro Mogi et al., “Preparation and Morphology of Triblock Copolymer of the ABC Type”, Macromolecules, vol. 25, p. 5408-5411 (1992), and Yushu Matsushita, Atsushi Takano, Naoya Torikai, and Atsushi Noro, “Molecular Design of Block—and Graft Polymers and Their Nanophase-Separated Hierarchical Structures In Condensed Systems”, Kobunshi Ronbunshu, Vol. 63, No. 4, p.205-218 (2006)).